NASA NEO News: Damage from small impacts

What would have been the consequences if 2004 AH had actually hit the
Earth instead of passing by at a distance of 50,000 km?

Stimulated by recent media attention on asteroids AL00667 and 2004
FH, there has been a lot of speculation about the damage a small
asteroid (of roughly 30 m diameter, kinetic energy about 1 megaton)
might cause if it hit the Earth. This is an important question, since
such "mini-Tunguska" impacts are the most frequent, happening once
every few decades. There has been a lot of confusion on this point,
however. Some have assumed that if it "disintegrates in the
atmosphere" it will do no harm, neglecting the fact that air-bursts
near the surface are exceedingly destructive, a fact well known to
the designers of nuclear bombs. At the opposite extreme, some assume
that an explosion of 1 megaton energy taking place anywhere in the
atmosphere is a major hazard, but this is true only if the explosion
occurs within about 10 km of the surface. Either conclusion - that
there is no danger or that such an impact would be equivalent to a
major nuclear strike - misrepresents the real situation.

It is very difficult to answer the seemingly simple question of what
would have been the consequence if 2004 AG had hit. There are many
sources of uncertainty, including (1) imprecision in the 30 m
estimate of diameter, (2) unknown physical and chemical nature of he
asteroid, and (3) dependence of damage on the target location. In
addition, there are uncertainties in the calculations, since the
nominal size of 30 m lies near the lower limit for penetration of the
atmosphere.

Let's look first at uncertainties in the calculations of atmospheric
penetration. A variety of models have been calculated for the effects
of the atmosphere on incoming projectiles, and these were reviewed
for the recent NASA Science Definition Team (SDT) report on
sub-kilometer asteroids. As an example, the 1908 Tunguska impactor
was a stony (asteroidal) object about 60 m in diameter. While it may
have been significantly decelerated in the upper atmosphere, the
Tunguska impactor still produced a very destructive explosion of
about 10-15 megatons energy when it disintegrated at an altitude of
6-8 km. In contrast, the energy of a 30 m stony asteroid at the same
speed is about 1 megaton, and it does not penetrate within 10 km of
the surface.

The studies reviewed by the SDT indicated that, for fixed composition
and entry conditions, the damage from a stony asteroid falls off very
rapidly for sizes smaller than Tunguska, going to zero for energies
below about 2 megatons. The drop-off is so fast toward smaller sizes
because two effects add: the energy is smaller and the explosion
takes place at higher altitude. If severe ground damage is defined by
a blast overpressure of 4 psi (pounds per square inch) as has been
standard in discussing the effects of nuclear explosions, then Al
Harris notes: "What is happening is that at 3 MT, the overpressure
falls under 4 psi at a distance of around 15 km from the blast
center, and the blast center is around 10 km or so up, so the
intersection with the ground is around 100 sq. km. If you drop the
size to 1 MT, the numbers are reversed: the blast is around 15 km up
and the radius out to 4 psi overpressure is around 10 km, so nothing
happens on the ground, even right below it."

One of the uncertainties is in estimating the damage for peak
pressures below 4 psi. My impression is that in the case of the 1 MT
explosion discussed above, we would expect a large sonic boom with
breakage of some windows near ground zero, but that buildings would
not generally suffer structural damage and trees would not be knocked
down. Flying debris from short-lived gale-force winds might cause
some injuries. The diameter for this 2-3 megaton limiting case is
near 40-45 m. The definitions of the Torino Scale for impact hazards
are more conservative, including any asteroid larger than 25 m as a
possibly harmful.

For purposes of estimating casualties, the NASA SDT concluded that
there is a significant atmospheric attenuation of the hazard for
impactors below about 70 m diameter (see their Figure 3-2), and that
there would be no deaths from an impact smaller than about 45 m
diameter. SDT member Al Harris stated this more colorfully by musing
that for a 30 m impact "I am about equally divided as to whether I
would run away from the impact site, or toward it. I seriously doubt
that any harm would come to anyone."

Damage from an airburst even as large as Tunguska (10-15 MT) depends
on the target. For the majority of the Earth's surface, which is
water, there would be no damage (the lower limit for tsunami
generation is about 10 times the Tunguska energy). Most of the land
is also still sparsely populated.

The larger source of uncertainty is the nature of the projectile
itself. Penetration and explosive energy will differ significantly
depending on the density and composition of the asteroid. At one
extreme, the rare iron object of this size makes it to the surface
and forms a crater - somewhere in the range between Meteor Crater in
Arizona and the cluster of small craters formed in the 1947
Sikhote-Alin strike in Siberia. At the other extreme, an ice-rich
impactor would detonate harmlessly at very high altitude. This
discussion has focused on the middle ground of stony objects because
they are by far the most numerous.

Also note that the astronomers do not measure the actual diameter of
the asteroid, but rather its apparent brightness. The diameter is
inferred from the brightness based on an assumed reflectivity or
albedo. Thus, even if we felt confident that a 30 m diameter stony
asteroid would do no harm, it does not follow that we can be sure
that asteroid 2004 FH would not be dangerous, since it could easily
be a factor of 3 larger or smaller than nominal in mass and energy.
Statistically, such asteroids do not constitute a significant threat,
as indicated in the NASA SDT report, where objects of this size are
concluded to contribute nothing to the hazard of sub-km asteroids.
However, that does not mean that we can ignore the possible danger
posed by a specific, poorly characterized object for which we have
measured only its brightness. Nevertheless, I would argue that it is
usually not appropriate to assume the worst case. By far the most
likely expectation for 2004 FH is negligible damage and zero
casualties if it had hit the Earth.

David Morrison

NEO News (now in its tenth year of distribution) is an informal
compilation of news and opinion dealing with Near Earth Objects
(NEOs) and their impacts. These opinions are the responsibility of
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